The exhaust emissions of passenger and commercial vehicles are the cause of various types of harm to the environment. The introduction of emission-limiting legislation has forced, and is forcing, vehicle manufacturers to reduce the emissions of individual vehicles byxe2x80x94for examplexe2x80x94developing advanced engines and exhaust systems.
One reason for a vehicle failing to conform to emission regulation is worsening performance, in terms of a gradual increase in exhaust emissions, as the vehicle ages. This is caused by wear and also, in part, by the incorrect functioning of components in the drive and emission-reduction systems.
The normal inspection procedure involves regular tests to attempt to keep emissions at or near their original level. The disadvantage of this method is that faults remain undetected until the next inspection, and excessive emissions meanwhile continue to be produced.
In the first few seconds after the engine is started, the catalytic converterxe2x80x94which has not yet reached its running temperaturexe2x80x94barely affects the level of harmful exhaust emissions. An engine produces about 70% of its total emissions just after starting from cold, so an ideal system for reducing harmful emissions would cover this phase, which is precisely the phase that remains untouched by current systems of emission control detection.
xe2x80x9cOn board diagnosisxe2x80x9d (OBD) is one new system for reducing harmful emissions. The term refers to an emission control system which uses sensors to monitor the performance of those individual components of a passenger or commercial vehicle that have a bearing on exhaust emissions. An early version of OBD for passenger carsxe2x80x94the OBD I Lawxe2x80x94has already been in use for a considerable time in the USA and is gradually being superseded by the more stringent OBD II Law for models from 1995 onwards. While OBD I only affected the performance monitoring of components forming part of an electronic engine control system, OBD II requires the control of all components relevant to emissions. The law expressly stipulates the monitoring of catalytic converters, lambda probes, fuel systems, air injection systems, exhaust gas recirculation, tank ventilation and the detection of misfiring. In the event of a component breaking down or malfunctioning, a warning lamp lights up on the dashboard and an error code is memorised. The fault detected should be located as precisely as possible and described. The information is then stored in order to permit swift identification of the fault at the workshop (using a standard interface) and to allow repair of the defective part.
A further step in this field is the use of xe2x80x9cOn Board Measurementxe2x80x9d (OBM). Systems for the direct analysis of vehicle emissions are widely familiar. Some examples, among many others, are the German public patents 32 32 416, 33 39 073, 36 08 122, 37 16 350, 39 32 838, 40 05 803, 41 24 116, 42 35 225, 43 07 190, the DE specification 43 19 282 C1 and U.S. Pat. No. 4,803,052 and 5,281,817, along with further patent applications GB 2 264 170 A, EP 0 196 993 A2 and WO 94/09266. These documents should be referred to for explanations in greater detail of items mentioned here.
Applications /1/ and /2/ are concerned, in a narrow sense, with subjects related to this field and are dealt with in greater detail for this reason. Thus patent /1/ describes an infrared measuring system that monitors the operating condition of the catalytic converter via a lateral access opening in the unit and measures the gases present inside. In /2/, the system in question is a rapid detector that uses several infrared cells connected in series to permit a chronological resolution of 0.1-0.2 sec. Neither source gives an indication of continuous measurement of harmful exhaust system emissions upstream of the catalytic converter.
Written sources reveal that no current measuring system is capable of providing a continuous record of actual emissions, either in the cold-start phase or during operation. Neither is it possible to detect fluctuations or indicate faults.
Vehicles will, in the future, be fitted with an integrated OBM system for the purposes of emission analysis. This system will analyse certain elements of the exhaust gases, and a comparison of current concentrations with a set of stored target values will permit the detection of faults in the ignition system. A warning system will then be activated whenever the xe2x80x9csatisfactoryxe2x80x9d level specified for the individual model of vehicle is exceeded, clearly and repeatedly, over a period. xe2x80x9cOver a periodxe2x80x9d means an extended length of time, xe2x80x9crepeatedlyxe2x80x9d signifies an excessive reading on not one, but various occasions and xe2x80x9cclearlyxe2x80x9d refers to a concentration that is outside the margin of tolerance specified.
FIG. 1 shows an example of how the concentration of harmful substances is affected by faults in the ignition system (caused by misfiring (1) in this case).
The measurement of emissions is hindered by the fluctuating conditions present in the vehicle. A measuring system must on one hand keep to the general margins of tolerance and specifications valid for the vehicle while, on the other hand, it is precisely the exhaust-related elements of pressure, moisture, temperature and flow rate that are subject to sharp fluctuations. In order to solve this problem, especially robust micro system components are requiredxe2x80x94both for exhaust gas processing and for the detection of the elements of which the gas consists.
One device for the analysis of vehicle exhaust gases is already familiar in the shape of DE 196 05 053 A1. Problems related to specified operation have however been encountered with this device, as they have with measuring devices described in other documents. The presence of vibrations in the vehicle requires that measuring systems be of highly stable construction and also resistant to soot, dust and aerosol precipitation. They must furthermore attain a high level of resolution, as the constituent components of the exhaust gas being analysedxe2x80x94e.g. carbon monoxide (CO), hydrocarbons (HC), and oxides of nitrogen (NO)xe2x80x94are present in extremely low concentrations, precisely in those petrol-driven vehicles that are fitted with a catalytic converter.
The emission analysis method used in the new OBM system submitted here is an infrared gas absorption process. This invention is based the assumption that, in order to obtain the required resolution, optical path length must be increased. The optical cell can thus be fitted to a vehicle if it is incorporated at the construction stage.